In recent years, demand for higher frequency operation and miniaturization has become further increased in electric communication apparatus and various electronic apparatus. To satisfy such demand, in addition to the utilization of overtone vibrations of conventional piezoelectric resonator such as a quartz vibrator, a surface acoustic wave (SAW) resonator is now being used in many fields of application.
The piezoelectric oscillator is designed such that a desired output is extracted through an LC resonance circuit that resonates to a desired overtone frequency or that a LC resonance circuit is inserted into a portion of an oscillation circuit to make the negative resistance of the oscillation circuit sufficiently large at a desired overtone frequency region. In any case, however, it is necessary to use a coil which is extremely inconvenient for the oscillation circuit to be incorporated into an integrated circuit.
On the other hand, as is well known in the art, the oscillation frequency of a SAW resonator is principally determined by the material constituting a piezoelectric substrate, and the pitch of interdigital transducer (IDT) electrodes formed on the surface of the piezoelectric substrate so that not only the resonator itself can be miniaturized but also above described circuit problems can be obviated. However, this type of resonator is defective in that its frequency-temperature characteristic is much inferior than that of an AT cut crystal resonator.
Factory workers manufacturing crystal vibrators have noticed a phenomenon that a crystal vibrators after oscillates in 3rd or 5th order overtones when the ratio between the length and the width of a crystal substrate, and the ratio of the side length with respect to the thickness of the substrate fall within a specified range as a result of miniaturization of the crystal substrate required for a small vibrator. To suppress such phenomenon has been subject to be studied.
U.S. Pat. No. 4,114,062 to Siemens discloses technology for utilizing this phenomenon so as to produce crystal vibrators for overtone oscillation.
The U.S. Patent, however, is unable to serve as a practical use because it has several problems as follows:
Considering a disc shaped substrate which is most typical shape for a vibrator substrate, the diameter of the substrate is extremely small, merely 20-30 times of the substrate thickness, and the diameter of the electrodes is about 50-90% of that of the substrate. Since the distortion of the support around the periphery of the substrate affects the oscillating portion, there arise problems that the frequency-temperature characteristic becomes unstable, resonant frequency varies before and after the thermo-cycle, and the resonant frequency greatly changes with its aging.
To solve these problems, U.S. Pat. No. 4,188,557 to Siemens proposes machining a piezoelectric substrate used in a small piezoelectric resonator for overtone oscillation into a convex shape.
However, merely entrapping overtone vibration energy of higher order than a desired order within excitation electrodes while forming a crystal substrate of a specific dimensional ratio, or merely providing a dot or line shaped dumping member at the periphery of the vibrator substrate is not sufficient at all for vibration energy whose overtone vibration order is lower than the desired order to be consumed. Accordingly, a vibrator of the above-mentioned type has a defect that it cannot provide a stable oscillation in overtone vibration of a desired order, and therefore no further study has been made on this type of vibrators.